1. Field of the Invention
This invention related to a method of producing by vacuum evaporation nanodimensional powders with particles enclosed in a sheath which protects them from the environment.
More specifically, this invention relates to a single-stage method of manufacture of encapsulated nanopowders of metals taken from the group consisting of Be, Mg, Al, Ca, Sc, V, Cr, Zn, Fe, Ni, Cu, Ti, Si, Co, Pt, Au, Zr, Mn, Sr, Y, Nb, Mo, Ag, Ba, La, Hf, Ta, W, Pt, Au, Bi, Ce, Sm and Dy or alloys on their base and chemical compounds of these metals with elements taken from the group consisting of B, C, O, Si (hereinafter referred to as “material”), encapsulated into a sheath of salt taken from the group consisting of NaCl, NaF, KCl, KF, LiCl and LiF and their mixtures (hereinafter referred to as “salt”) using evaporation of material and salt in a closed volume and combined condensation of their vapor phases on a metal substrate.
2. The Prior Art
It is known that powdered materials change greatly as to their functional properties, such as melting temperature, heat of evaporation, ionization energy, work function of electrons, magnetic properties, etc., when sizes of separate particles reach a nanodimensional scale (sizes of particles become less than 100 nm) [A. I. Gusev, A. A. Rempel, Nanocrystalline materials, Moscow, Phizmatgiz, 2001; R. A. Andrievsky, A. M. Gleser, Physika metallov i metallovedeniye, 1999, v. 88, No. 1, p. 50-73]. Owing to specific properties of nanopowders, and also to the nanodimensional scale of separate particles, the nanopowder materials find a wide application as functional materials with special magnetic and catalytic properties, energy activators of sintering processes and fillers for lubricating materials. In addition, the application of nanopowders opens up wide opportunities in the field of development of new materials and technologies and radically new instruments and devices. Thus, the nanopowders have found wide use as initial raw material in the production of nanocomposite materials, superconductors, solar batteries, filters, getters, components of low-temperature high-strength solders and other materials, whose high service properties are attained due to decrease in grain size.
Taking the above into account, the technology of manufacture of nanopowder materials is key technology for the solution of a wide range of problems. Moreover, it should provide their high resistance, despite the structural and chemical homogeneity of such powders, to their agglomeration and interaction with the atmosphere. One of the ways to achieve this aim is the encapsulation of the nanopowder particles.
The main methods of producing a nanopowder are the method of a gas phase and the method of a liquid phase [M. T. Swihart, Current Opinion in Colloid and Interface Science, v. 8, (2003), p. 127-133]. In case of a method of a gas phase, the nanopowders are produced by evaporation-condensation under conditions of a rapid cooling, while in the application of the liquid-phase method, the synthesis, for example, of metal nanopowders occurs with the use of organic solvents and polymers.
When producing nanopowders with participation of a gas phase, the material evaporation is realized by different methods; i.e., in a plasma jet, heating by electric current, laser or electron beam and others. However, these methods do not provide the necessary conditions for prevention of change in chemical composition of powders, for example, oxidation of the surfaces of particles of a metal powder and their agglomeration with time. Moreover, an explosion is possible at a definite concentration of powder particles in the atmosphere. Taking into account the pyrophoric nature of powders and their high energy saturation, the obligatory condition in the use of such methods is the encapsulation of particles of a nanopowder into a protective sheath which prevents their agglomeration and interaction with the environment (atmosphere) or conductance of synthesis of nanoparticles in the medium in which the synthesized particles are preserved. In known methods with participation of a liquid phase, the polymers, organic and inorganic salts are used to prevent the agglomeration of particles.
Thus, in patent No. WO2004/078641 of Sep. 16, 2004, Cl. IPC B82B3/00; B82B3/00; (IPCl-7); B82B3/00 (KANG DAE SAM), a liquid phase method is described for producing a metal nanopowder, encapsulated into a silicon oxide. The process consists in a combined disposition of metal ions, located in solution, and particles of silicon oxide, produced as a result of hydrolysis of an appropriate solution, on the electrode surface. Taking into account the high chemical resistance of the silicon oxide, the drawback of this method is the difficulty in the particles' release from the sheaths, into which they are encapsulated, for further use. In U.S. Pat. No. 5,593,740 of Jan. 14, 1997, Cl. IPC B22F9/12; C23C14/00; C23C14/32; C23C16/26; H05H1/24; B22F9/02; C23C14/00′ C23C14/32; C23C16/26; H05H1/23; (IPCl-7); H05H1/24 (STRUMBAN EMIL E., et al.), a method of synthesis of carbon-encapsulated metal nanopowder, produced by evaporation in a plasma jet, is suggested. The process consists of the following stages: metal is evaporated in arc melting and mixed with a gas, which contains carbon. The produced mixture is cooled at high rate. This results in the formation of metal nanoparticles in a sheath, containing a carbon.
The drawback of this proposed method of producing encapsulated nanoparticles is the difficulty in separation of nanoparticles from the sheaths covering them, which are formed on the base of inorganic elements of a high inertia as regards to the known solvents.
The method of producing a nanopowder, encapsulated in a salt matrix, which ensures its resistance to oxidation and is most close by combination of essential features to the present invention and selected as a prototype, is that described in patent No. WO9532061 of Nov. 30, 1995, Cl. IPC B01J2/00; B22/F1/02; B22F9/28; C01B113/22; C0135/04, C01G23/07; C23C16/30; C23C16/453; C23C16/56; B01J2/00; B22F1/02; B22F9/16; C01B13/20; C01B35/00; C01G23/00; C23C16/30; C23C16/453; C23C16/56; (IPCl-7): B05D7/00; C23C16/00; F23J7/00 (AXELBAUM RICHARD L. et al.). According to the description of the invention there, the nanopowder, whose encapsulated particles represent metals or intermetallic compounds, was produced by evaporation of metal and salt components in a closed volume, mixing of vapor phases of the pre-set components and their subsequent condensation. The closed volume of the reactor was filled preliminarily with argon, the evaporation of metal and salt components occurred in a gas flame with their subsequent condensation in the reactor atmosphere in the presence of a high degree of vapor oversaturation in the reactor atmosphere. As a result of this, the metal particles, encapsulated in a salt (sodium chloride), are formed. It follows from the invention description there that the salt matrix limits the agglomeration of nanoparticles and prevents their oxidation during their removal from the reactor into atmosphere. Nevertheless, this method does not allow producing nanopowders with a high degree of chemical purity, as the process of formation of powder particles occurs in the medium, saturated with gases and combustion products. Moreover, taking into account that the process of a particle's origin and its growth can occur in the reactor regions with a different degree of saturation with vapors, temperature, etc., this method makes it impossible to control the sizes of particles and to provide the formation of a nanopowder with a small dispersion of distribution of particles by size.